Chemistry Reference
In-Depth Information
The stabilizing effects of delocalization and lone pair repulsion are significantly diminished beyond
the first row of the periodic table. In the second row of the p-block and beyond, the larger size of the
valence orbitals makes for more diffuse lone pairs; the alpha effect in heavier atom chemistry is not well
documented but is anticipated to be less important than is known for nitrogen and oxygen. In addition,
most heavy main group elements show a strong preference for
bonding.
2
As a result, the many
different examples of stable (isolable) radicals based on heavy p-block elements rely heavily on bulky
substituents for their kinetic and thermodynamic stability.
3
In the absence of steric bulk, most radicals in
which substantial spin density is located on a heavy main group element are not stable or even persistent.
The lone and dramatic exception is sulfur, specifically when linked to nitrogen. To be sure, the differences
in atomic properties of sulfur and oxygen (atomic radius, electronegativity,
σ
π
over
energetic) do produce
distinctive chemistries in many respects, including in the structures and stabilities of radical species. The
most fundamental distinction can be found in the common forms of the two elements (O
2
vs S
8
), and the
stable open shell species O
2
, NO, and NO
2
have no counterparts in sulfur analogues as stable entities. The
dissimilarities between the two chalcogens is further demonstrated by the fact that while stable nitroxides
R
2
NO
•
are ubiquitous, the corresponding thionitroxides R
2
NS
•
are not stable and dimerize to disulfides
R
2
NSSNR
2
.
4
However, the thiazyl (herein defined as an unsaturated SN unit in which both sulfur and nitrogen
are two-coordinate)
σ/π
bond is of considerable strength and it is instructive to compare this fundamental
building block of thiazyl chemistry with a carbon-carbon
π
bond (Figure 9.1).
5
The -S
=
N-moiety has a
π
three-electron
* orbital. Conjugated species containing multiple SN
units have often been referred to as “electron rich” because as they have more
π
bond, with the third electron in a
π
electrons than atoms.
6
However, the higher electronegativity of both sulfur and nitrogen relative to carbon provides some stability
to these “electron rich” species. For example, the cyclic S
3
N
3
anion (Section 9.2.2) is a perfectly stable,
π
CC
N
S
(a)
(b)
Figure9.1
Qualitative
π
molecularorbitaldiagramsforC
2
H
4
(left)andHSNH(right).
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